Analog to digital conversion with charge balanced voltage to frequency converter having polarity responsive offset

ABSTRACT

A bipolar analog voltage is converted into a digital signal by sensing the polarity of the voltage and selectively supplying a bias voltage to an analog-to-digital converter, which can preferably be a charge balanced voltage to frequency converter, as a function of the sensed polarity. The voltage to frequency converter has a double valued variable frequency output with a discontinuity at zero volt such that the converter derives a maximum output frequency for a maximum positive voltage and also for a negative value slightly displaced from zero; the voltage to the frequency converter minimum output frequency is derived from positive voltages slightly greater than zero and for maximum negative voltages. The converter output frequency and the sensed polarity are supplied to a frequency to digital converter which derives an output signal having a bit representing the polarity of the analog voltage and additional bits indicative of the magnitude of the analog voltage.

FIELD OF INVENTION

The present invention relates generally to analog to digital convertersand more particularly to an analog to digital converter that is made tobe responsive to bipolar input voltages by means of a bias voltagecoupled to the input of the converter. The invention has particularapplication to charge balanced voltage-to-frequency converters.

BACKGROUND ART

Among the useful attributes of charge balanced voltage-to-frequencyconverters is the fact that they can be used to implementanalog-to-digital converters. Charge balanced voltage-to-frequencyconverters generally include a capacitor, connected with an operationalamplifier to form a current integrator, that is cyclically charged,first, in one direction and, second, in the opposite direction (i.e.,charged and discharged). This is done at a frequency which changeslinearly with the input voltage applied. The net charge applied to thecapacitor during each cycle is zero, a result achieved by charging thecapacitor in the first direction for a predetermined time. In responseto the end of that time, the capacitor is charged in the second(opposite) direction until a predetermined level is reached. The rate atwhich the capacitor is charged in the first direction is controlled bythe sum of a current derived from the input voltage and a fixed currentsource. The rate at which the capacitor is charged in the seconddirection is determined by a current derived from the input voltage.Therefore, the frequency of the charge and discharge cycles is a directfunction of the input voltage magnitude.

Among the well known advantages of charge balanced voltage-to-frequencyconverters are: (1) the inherent filtering provided by the capacitorwhich is connected to and responsive to the input during the entirecharge/discharge cycle, and (2) the fact that this eliminates the needfor an anti-aliassing filter in systems where one is digitizing theinput in discrete samples.

Several different techniques have been employed for enabling chargebalanced voltage-to-frequency converters to handle bipolar inputvoltages. One prior art bipolar charge balanced voltage-to-frequencyconverter includes an absolute value circuit connected between thevoltage source and the charge balanced converter circuit. In anidealized situation a converter with an absolute value circuit respondsto input voltages of -10 volts, 0 volt and +10 volts to derivefrequencies of (for example) 100 kHz, 10 kHz and 100 kHz, respectivelyThe negative voltage levels are detected to control a polarity bit whichis combined with the counted frequency, containing the magnitudeinformation, to determine the complete reading. However, the absolutevalue circuit has a tendency to have slightly different gain factors forpositive voltages relative to negative voltages. The different gainfactors of the absolute value circuit introduce errors in the voltageversus frequency relationship of the converter so that, in the aboveexample, the derived frequencies may be 100 kHz, 10 kHz and 99 kHz.Further, absolute value circuits with even the above inaccuracygenerally require the use of high accuracy operational amplifiers whichare slow to settle. The settling time required for a circuit combinedwith the preamplifier ahead of the voltage to frequency converter caneasily be 100 microseconds. This could be shortened greatly byimplementing the absolute value circuit after the preamplifier, but atthe expense of added parts, hence added cost and error.

Another bipolar charge balanced voltage-to-frequency converter providesa bias to the converter, such that a zero voltage input is offset to apredetermined value at the input of the converter. The maximum negativeand positive voltages applied to such converters result in the converterderiving minimum and maximum frequencies, respectively, while a zeroinput voltage results in the converter deriving a median outputfrequency. For example, the bias applied to such a converter causes theconverter response to be 10 kHz, 55 kHz and 100 kHz, respectively, forinput voltages of -10 volts, volt and +10 volts. Such converters havethe advantage of simplicity over converters which use an absolute valuecircuit, but they sacrifice one bit of resolution when compared tounipolar converters or those which use an absolute value circuit. Thislost bit of resolution means that such converters have only one-half theresolution that the other converters have. Thus, the accuracy of priorart converters using a built in bias is reduced considerably.

A third prior art bipolar charge balanced converter includes a pair ofconverters, one for positive voltages and a second for negativevoltages. Such a construction is disadvantageous because of the doubledcost of two converters and the difficulties in obtaining two convertershaving exactly the same responses to voltages of the same amplitude butof opposite polarity.

It is, accordingly, an object of the present invention to provide a newand improved bipolar charge balanced voltage-to-frequency converter.

Another object of the invention is to provide a new and improved, highaccuracy, high resolution charge balanced voltage-to-frequencyconverter.

Another object of the present invention is to provide a new andimproved, relatively inexpensive and highly accurate charge balancedvoltage-to-frequency converter.

DISCLOSURE OF THE INVENTION

In accordance with the present invention a charge balancedvoltage-to-frequency converter responsive to bipolar voltages senses thepolarity of the input voltage and selectively connects a bias currentderived from a bias voltage to an input of the charge balanced converteras a function of the sensed polarity. The bias current which isselectively applied as a function of the polarity of the input voltagebeing converted has a polarity opposite that of a constant currentsource applied to the converter periodically to balance the chargeapplied to a capacitor.

In a preferred embodiment of the invention, a circuit for converting avoltage having positive and negative (opposite) polarities to an outputfrequency that is related to the value of the voltage and wherein thevoltage magnitude can include zero, comprises a capacitor connected tothe inverting input of an operational amplifier to implement anintegrator. The input voltage, and all other voltages used, are appliedto the integrator through resistors which effectively turn them intocurrent sources charging or discharging the capacitor while theoperational amplifier holds the voltage of its inverting input terminalat a constant voltage that is very near ground potential. In response tothe input voltage having a positive polarity the capacitor is cyclicallycharged and discharged, i.e., charged in a first direction and then in asecond, opposite, direction at a first rate determined by a constantcurrent source and a current derived from the input voltage and at asecond rate determined by a current derived from the input voltage, sothat zero net charge is applied to the capacitor during each cycle.While the input voltage has a negative polarity, the capacitor ischarged at a third rate, determined by the sum of a current derived fromthe input voltage and a current derived from an offsetting voltagesource and a constant current source, and discharged at a fourth rate,determined by a current derived from the input voltage, and a currentderived from the offsetting voltage. The variable capacitor charge ratesdetermine the frequency of a triangular-like wave derived from theconverter. The frequency of the triangular-like wave is counted and usedin combination with the sensed polarity to derive an accurate indicationof the voltage magnitude and polarity.

The input voltage magnitude versus output frequency relationship of theconverter is monotonic for each polarity. However, the input voltagemagnitude versus output frequency is double valued when both polaritiesare considered, i.e., the output frequency has the same value for onepositive voltage as for one negative voltage. This increases the inputvoltage to output frequency resolution of the converter by a factor oftwo. Accordingly, greater accuracy is achieved without the inherentdisadvantage of an absolute value circuit and without the need forduplicated converters for the two polarities, merely by properly settingthe magnitude of a bias voltage which is applied to the converter as afunction of input voltage polarity.

In the preferred embodiment, there is a step function change in outputfrequency for input voltage in the vicinity of zero volt. This isachieved by simply switching an offsetting current derived from anoffsetting voltage source in or out of the circuit. This is highlyadvantageous because the percentage error in the vicinity of zero voltis kept small, which is not the case in other implementations of bipolarvoltage-to-frequency converters. In the preferred embodiment, forvoltages varying between zero and +Vmax the converter output frequencyvaries from F1 to F2, where F1 is less than F2 and is preferably notzero because of the need to obtain an accurate count of it within ashort time, i.e. in practice less than 500 microseconds. For inputvoltage between zero and -V, the output frequency varies from F2 to F1.

In accordance with another invention a bipolar analog to digitalconverter comprises a polarity sensor responsive to the analog voltageand a voltage-to-frequency converter responsive to the sensed polarityand the analog signal for deriving a variable frequency signal havingtwo frequency values as a function of the polarity of the analog signal.There is a discontinuity in the frequency derived by thevoltage-to-frequency converter as the analog signal changes polarity Afrequency to digital converter responds to the variable frequency outputof the voltage-to-frequency converter and to the polarity sensor forderiving a binary representation having a polarity bit indicative of thesensed polarity and remaining bits controlled by the frequency derivedby the voltage to frequency converter and the sensed polarity.

It is, accordingly, still another objective of the invention to providea new and improved analog-to-digital converter employing a chargebalanced voltage-to-frequency converter responsive to bipolar inputvoltages.

A further objective of the invention is to provide a new and improvedbipolar analog-to-digital converter that is highly accurate andsensitive to voltage variations in the vicinity of zero volt.

A further objective of the present invention is to implement ananalog-to-digital converter which can maintain a high level of accuracyand resolution while requiring only a short time to sample the input.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an analog-to-digital converter employinga charge balanced voltage-to-frequency converter in accordance with thepresent invention;

FIG. 2 is a drawing of the input voltage-output frequency response ofthe charge balanced voltage-to-frequency converter employed in theanalog-to-digital converter of FIG. 1; and

FIGS. 3(a) and 3(b) are a series of waveforms helpful in analyzing theoperation of the circuit of FIG. 1.

DETAILED DISCLOSURE OF THE INVENTION

Reference is now made to FIG. 1, wherein bipolar analog voltage source11 generates an output that is connected to an input of ananalog-to-digital converter 12 including charge balancedvoltage-to-frequency converter 13, polarity detector 14, control logicnetwork 15 and frequency and polarity to digital converter 16. Polaritydetector 14 is connected directly to the output of source 11 to derive abinary signal indicative of the polarity of the voltage present at theinput.

Charge balanced voltage-to-frequency converter 13 responds directly tothe output of the source 11 and to the polarity indicating signalderived from the detector 14 to derive a triangular wave output havinglinear variations such that in response to the voltage of source 11having a positive polarity the converter derives a waveform thefrequency of which increases linearly with increasing input voltage fromsource 11; in response to negative polarity voltages from source 11 theconverter derives a waveform the frequency of which decreases linearlywith increasing magnitude of input voltage from source 11. Referencevoltage source 28 and resistor 120 provide a constant bias current tothe input of the converter which ensures that the minimum slope of thewaveforms, and hence the minimum frequency of the converter, is alwaysgreater than zero, preferably, near 10 kHz. Comparator 24 and one-shotmultivibrator 25 respond to the triangular wave to generate outputpulses having a frequency equal to the triangular wave frequency.Frequency and polarity to digital converter 16 responds to variablefrequency and polarity input signals to derive a digital representationhaving a polarity bit representing the polarity of source 11 andadditional bits representing the magnitude of the voltage of the source11. Control logic circuit 62 supplies signals to polarity detector 14and converter 16 to control the operation thereof.

Charge balanced voltage-to-frequency converter 13 includes an analogintegrator comprising resistor 21, operational amplifier 22 and feedbackcapacitor 23. Resistor 21 is connected between an output terminal ofvoltage source 11 and the inverting input terminal of amplifier 22,while capacitor 23 is connected between an output terminal of theamplifier and the inverting input terminal of the amplifier. The outputof operational amplifier 22 is applied to an input of comparator 24,having a second, grounded input whereby the comparator derives apositive voltage signal in response to the output of amplifier 22 beingequal to or less than zero. At all other times comparator 24 derives azero output voltage.

The signal derived by comparator 24 is applied to the input of amonostable multivibrator (one-shot flip-flop) 25, whereby the one-shotderives a bilevel output signal having a binary one positive level for apredetermined interval of time after comparator 24 supplies it with apositive transition. At all other times, the output of one-shot 25 is abinary zero, ground level. The output of one-shot 25 is supplied as acontrol input to switch 26 so that the switch is opened and closed whilebinary zero and one levels are respectively derived from one-shot 25.Switch 26 is connected in series between the output of a negativeconstant current source 27 and the inverting input terminal ofoperational amplifier 22.

In response to a positive output voltage from voltage source 11,capacitor 23 is cyclically charged and discharged at first and secondpredetermined rates, where the first rate is determined by the sum ofthe currents caused by the voltage from source 11, applied acrossresistor 21, and the constant current from source 27 (while switch 26 isclosed). The second rate is determined only by the voltage from source11 applied across resistor 21. While switch 26 is closed, the netcurrent applied to the inverting input of amplifier 22 is negative basedon the selection of constant current source 27 to have a magnitudegreater than or equal to the maximum current from source 11 appliedacross resistor 21.

In response to the net negative current supplied to the inverting inputterminal of amplifier 22 from sources 11 and 27, the amplifier derives apositive going, linear ramp output voltage by virtue of capacitor 23being charged by the output of amplifier 22. The positive going linearramp is maintained for a predetermined time interval, determined by theduration of the binary one output of one-shot 25, which in turndetermines the length of time switch 26 is closed.

In response to the output of one-shot 25 returning to the binary zerolevel, switch 26 is opened, causing a positive net current to besupplied by source 11 through resistor 21 to the inverting input ofamplifier 22. The net positive current applied to the inverting input ofamplifier 22 is inverted by the amplifier into a negative voltage rampat its output that decreases linearly by virtue of capacitor 23 beingdischarged. In response to the negative going voltage at the output ofamplifier 22 reaching a zero level, comparator 24 derives a positiveoutput transition which triggers one-shot 25, causing the one-shot toderive a binary one level for the previously mentioned predeterminedinterval to define the next cycle. The triangular wave voltage at theoutput of amplifier 22 thus has a variable frequency determined by themagnitude of voltage source 11.

From the foregoing, the charge applied to and removed from capacitor 23during each operating cycle has a net value of zero, leading to the name"charge balanced voltage-to-frequency converter". While switch 26 isopen, the only input current supplied to the inverting input terminal ofamplifier 22 is from source 11, through resistor 21. Thus the onlyvariable affecting the charge and discharge rates of capacitor 23 andthereby the slopes of the output of amplifier 22 during the charge anddischarge times of the capacitor are indicative of the voltage of source11 while source 11 has a positive value.

The previously described elements of charge balancedvoltage-to-frequency converter 13 are well known, as is the operationthereof. In accordance with the present invention, the charging rate ofcapacitor 23 is modified by connecting a positive reference voltagesource 28 to the inverting input of amplifier 22 through resistor 121when the voltage from source 11 is negative. Such a modification enablescharge balanced voltage-to-frequency converter 13 to operate over thesame range as if it were responding to voltages of only positivepolarity from source 11. The magnitude of the current supplied to theinverting input terminal of amplifier 22 while the polarity of source 11is negative is determined by setting the value of resistor 121 to equalthe value of resistor 21 and the voltage magnitude of reference voltagesource 28 to equal Vmax, where Vmax equals the maximum voltage of source11. Setting R121=R21 and Vref=Vmax is not fundamental to the invention,although implementation is simplified thereby. Selection of themagnitude of the current supplied by the reference voltage source 28while the voltage of source 11 is negative to correspond to Vmax ensuresthat capacitor 23 is cyclically charged and discharged in the samemanner and at the same rates for negative voltages from source 11 as itwould be for positive voltages from the source 11.

To these ends, charge balanced voltage-to-frequency converter 13 is incircuit with reference voltage source 28, resistor 121 and switch 29connected between the output of source 28 and one end of resistor 121.The other end of resistor 121 is connected to the inverting inputterminal of amplifier 22. Source 28 provides a constant current outputthrough resistor R121 based on the ability of amplifier 22 to hold itsinverting input terminal at a constant voltage, very near groundpotential, by supplying current to that terminal through capacitor 23.The sum of the currents from source 11 and reference voltage source 28never exceeds Vmax/R21 because voltage source 28 is applied only whenvoltage source 11 derives a negative value. Therefore the operation ofthe voltage-to-frequency converter with negative voltages applied fromsource 11 and with reference voltage source 28 applied through resistor121 is substantially the same as its operation with positive voltagesapplied from voltage source 11 without reference voltage source 28applied. However, it is important to recall that the application ofreference voltage source 28 does not attempt to invert thevoltage-to-frequency characteristic as an absolute value circuit would,but rather offsets the characteristic. It is also important to note thatapplying offsetting reference voltage source 28 does not cause theamplifier 22 input voltage or output voltage to change, which wouldrequire significant settling time, e.g., up to 100 microseconds for a 1MHz amplifier. Rather, it causes a change in the rate at which theoutput voltage of the amplifier 22 changes, which takes place in a muchshorter time, e.g., on the order of 1 microsecond for a 1 MHz amplifier.

While switch 29 is closed in response to a negative output voltage fromsource 11, and while switch 26 is closed in response to a binary oneoutput level of one-shot 25, capacitor 23 is charged, resulting in apositive going linear voltage ramp being derived at the output ofoperational amplifier 22. In response to switch 26 being opened at theend of the predetermined time interval for the binary one output ofone-shot 25 being completed, only the negative current supplied fromsource 11 through resistor 21 and the positive current of source 28,coupled through switch 29 and resistor 121, flow into the invertinginput terminal of amplifier 22, resulting in a negative going outputlinear voltage ramp at the output terminal of the amplifier 22. When thenegative going output voltage reaches a zero level, comparator 24triggers one-shot 25, completing the cycle.

For a given magnitude of voltage source 11, the charge and dischargerates of capacitor 23 may differ. They combine to form a triangular waveform which has a frequency that corresponds to that voltage value fromsource 11 and causes one-shot 25 to output pulses at that samefrequency. The relationship between the output frequency ofvoltage-to-frequency converter 13 and the magnitude and polarity of thevoltage from source 11 is illustrated in FIG. 2. Therein, as the voltageof source 11 increases from zero volt in a positive direction to themaximum value of +Vmax, the frequency derived from voltage-to-frequencyconverter 13 increases in a linear, monotonic manner from fmin to fmax,as indicated by straight line 31. In response to the voltage of source11 increasing from zero volt to -Vmax, the output frequency ofvoltage-to-frequency converter 13 decreases in a linear, monotonicmanner from fmax to fmin, as indicated by straight line 32. In a typicalexample, the values of fmin and fmax are respectively 10 kHz and 100kHz, while the value of Vmax is 10 volts. In any event, the value offmin is deliberately maintained greater than zero by the offset currentthat flows from reference voltage source 28 through resistor 120 intothe inverting input of the operational amplifier 22.

From the foregoing, it is apparent that there is a significant, finitejump in the output frequency of voltage-to-frequency converter 13 in thevicinity of zero volt, as a function of the polarity of the voltagederived by source 11, sensed by polarity detector 14.

The triangular waveforms derived by amplifier 22 withinvoltage-to-frequency converter 13 in response to output voltages fromsource 11 being equal to Vmax/4 and -Vmax/4 are illustrated in FIGS.3(a) and 3(b), respectively. While switch 26 is closed, capacitor 23 islinearly charged as indicated by wave segment 33 in FIG. 3(a) at a ratedetermined by the value of (I27-Vmax/4R21-Vmax/R120), where I27 is thecurrent from source 27 and the output of amplifier 22 increases for apredetermined time T1 equal to the time interval while the one-shot 25is deriving a binary one output After time T1 has expired, the outputvoltage of amplifier 22 linearly decreases from V1 to zero, as indicatedby waveform segment 34. The slope of waveform segment 34 is determinedsolely by the value of (-Vmax/4R21-Vmax/R120). From the foregoing, it isappreciated that the duration of waveform segment 34 is variable, and isthe primary determinant of the frequency of the triangular wave outputderived by amplifier 22 and the frequency of pulses derived fromone-shot 25.

If the voltage of source 11 changes while the polarity of the sourceremains positive, the maximum voltage of the triangular wave and thefrequency of the triangular wave also change. As the magnitude of thepositive voltage of source 11 decreases, the maximum voltage of thetriangular wave increases, accompanied by a slight increase in the slopeof the positive going wave segment 33 and a significant decrease in theslope of the negative going wave segment 34. Thus, segment 34 requires agreater time to reach zero volt, reducing the frequency of thetriangular waveform shown. Because the time interval spanned by wavesegment 33 is constant and determined by one-shot 25, the slope ofsegment 33 does not affect the frequency of the waveform directly.However, the less steep slope of segment 34 causes it to reach thepredetermined level of its completion later, resulting in a longerperiod of the waveform and a lower frequency. Therefore, as themagnitude of the positive voltage of source 11 decreases, the frequencyof the triangular wave decreases, despite the constant interval ofpositive going wave segment 33.

Now consider the triangular wave output of amplifier 22 in response tosource 11 deriving a negative output voltage having a magnitude Vmax/4,as illustrated in FIG. 3(b). During the charge portion of each cycle, asindicated by straight line waveform segment 35, switches 26 and 29 areclosed, whereby the net current flowing to the inverting input terminalof amplifier 22 from sources 11, 27 and 28 is(-Vmax/4R21-I27+Vmax/R120+Vmax/R121), where Vmax/R121 is the currentthat flows from reference voltage source 28 through resistor 121 andswitch 29. Since I27 is much greater than all the other currentstogether, the output voltage of amplifier 22 increases linearly fromzero value for a predetermined length of time determined by one-shot 25at a slope determined by the net current, as indicated by wave segment35. In response to switch 26 being opened in response to the expirationof time interval T1, the capacitor 23 is discharged to zero volt at alinear rate determined by (-Vmax/4R21+Vmax/R120+Vmax/R121), whereby theslope of negative going linear discharge segment 36 in FIG. 3(b) isgreater than the slope of waveform segment waveform 34 in FIG. 3(a).Waveform segment 36 continues until a zero level is detected bycomparator 24, causing triggering of one-shot 25 to close switch 26,completing the cycle. Because of the larger slope of waveform segment 36than waveform segment 34, the frequency of the triangular wave output ofamplifier 22 and of pulses derived by one-shot 25 is greater for thevoltage of source 11 being -Vmax/4 than for the voltage of source 11being +Vmax/4.

Polarity detector 14 includes a voltage comparator 41 having a signalinput terminal connected to be responsive to the output of bipolaranalog voltage source 11 and a second, grounded input terminal. Inresponse to the input voltage to comparator 41 from source 11 beingequal to or greater than zero, the comparator derives a binary zerooutput level; in response to the voltage of source 11 being negative,comparator 41 derives a binary one output level.

The binary level derived by comparator 41 is sampled by D flip flop 42once, shortly after the voltage from source 11 has been applied butbefore the frequency-to-digital converter 16 has begun counting theoutput frequency of the converter 13. The Q output of flip-flop 42 issupplied as a control signal for switch 29 and as a polarity bit tofrequency-to-digital converter 16.

Frequency to digital converter 16 includes a counter 50 which counts thefrequency of a local oscillator 61 that operates at a much higherfrequency than the maximum frequency from the voltage-to-frequencyconverter 13, e.g., 40 MHz and counter 51 which counts the pulses fromone-shot 25. Counter 50 begins counting high frequency clock pulsesimmediately after a high-to-low transition of the output of one-shot 25,responding to a predetermined number of clock pulses to establish aminimum sample time. During that sample time, counter 51 countssubsequent high-to-low transitions of one-shot 25. At the end of thepredetermined sample time, counter 50 is reset to zero, or simply "rollsover", and begins counting up again. The counter 50 continues countinguntil the next high-to-low transition of the output of one-shot 25,which stops both counters 50 and 51. At this time, counter 51 contains anumber which corresponds to an exact number of frequency cycles of thevoltage-to-frequency converter, while counter 50 contains a number whichwill be added to the predetermined number at which counter 50 was resetto zero, to determine the length of time required for the whole numberof voltage-to-frequency cycles to occur. By dividing the number ofcycles from counter 51 by the length of time required for those cyclesfrom counter 50, the frequency of the voltage-to-frequency converter isdetermined with a high degree of resolution within a few cycles. Thismethod could be used with very high local oscillator frequency to obtainthe desired frequency data within one cycle of the voltage-to-frequencyconverter. In practice, this is limited by phase noise of thevoltage-to-frequency converter 13 and the maximum clock frequency atwhich counter 50 operates.

Division of the number of cycles from counter 51 by the length of timederived by counter 50 may be performed either in hardware or insoftware. The polarity bit is then mathematically combined with theresult of the division to map the frequency into the correct positive ornegative number corresponding to the voltage provided by source 11.Again, this mapping may be done in either hardware or software. Oncethis mapping is done, the polarity bit and the number corresponding tothe voltage magnitude are available as a reading of the input voltagefrom source 11.

While there has been described one specific embodiment of the invention,it will be clear that variations in the details of the embodimentspecifically illustrated and described may be made without departingfrom the true spirit and scope of the invention as defined in theappended claims.

We claim:
 1. A circuit for converting an input voltage having first andsecond opposite polarities to an output frequency wherein the outputfrequency is related to the value of the input voltage, the inputvoltage value including zero, the output frequency having a non-zerovalue for a zero input value, comprisinga capacitor; means responsive tothe input voltage for cyclically charging the capacitor in first andsecond opposite directions so that zero net charge is applied to thecapacitor during each cycle, the means for charging including means forcharging the capacitor (a) in the first direction at a first ratedetermined by the value of the input voltage and a predetermined currentwhile the input voltage has the first polarity, (b) in the seconddirection at a second rate determined by the value of the input voltagewhile the input voltage has the first polarity, (c) in the firstdirection at a third rate determined by the value of the input voltage,a predetermined voltage and the predetermined current while the inputvoltage has the second polarity and (d) in the second direction at afourth rate determined by the value of the input voltage, thepredetermined voltage while the input voltage has the second polarity,the predetermined voltage and predetermined current being of oppositepolarity; and means responsive to the charging and discharging of thecapacitor for deriving the output frequency.
 2. The circuit of claim 1wherein the means for cyclically charging includes: means for supplyinga predetermined constant current having the second polarity to thecapacitor while the capacitor is being charged in the second directionfor a predetermined interval during each cycle and means for supplying apreset constant current having the first polarity to the capacitorthroughout each cycle while the input voltage has the second polarity,the magnitude of the predetermined current being equal to the maximumcurrent flowing from the source of the input voltage to the capacitor,the magnitude of the predetermined current exceeding the magnitude of amaximum current derived from the predetermined voltage.
 3. The circuitof claim 1 further including means for sensing the polarity of the inputvoltage at a predetermined time, said voltage polarity sensing meansbeing connected to said charging means to control whether the capacitoris charged at the first and second rates or at the third and fourthrates.
 4. A circuit for converting an input voltage having first andsecond opposite polarities to an output frequency wherein the outputfrequency is related to the value of the input voltage, the inputvoltage value including zero, the output frequency having a non-zerovalue for a zero input voltage value, comprising a charge balancedvoltage to frequency converter responsive to the input voltage, andmeans responsive to the input voltage having the first and secondpolarities for supplying a predetermined bias voltage having the firstpolarity only when the input voltage is at the second polarity.
 5. Ananalog to digital converter responsive to an input voltage source havingfirst and second polarities and a predetermined amplitude range,comprising means for converting the input voltage into an outputfrequency, the output frequency varying as a function of the amplitudeand polarity of the input voltage such that there is a step functionchange in the output frequency with changes in polarity of the inputvoltage, the output frequency having the same value for two differentmagnitudes and polarities of the input voltage, and means responsive tothe polarity of the input voltage and the output frequency for derivinga digital signal having different values for different voltagesthroughout the range.
 6. The converter of claim 5 wherein the means forconverting includes a charge balanced voltage to frequency converterresponsive to the input voltage, and means responsive to the inputvoltage having the first and second polarities respectively causingfirst and second polarity currents to flow to the converter and forselectively supplying a predetermined bias current having a firstpolarity to the converter.
 7. The converter of claim 5 wherein the meansfor converting includesa capacitor; means responsive to the inputvoltage for cyclically charging the capacitor in first and secondopposite directions so that zero net charge is applied to the capacitorduring each cycle, the means for charging including means for chargingthe capacitor (a) in the first direction at a first rate determined bythe value of the input voltage and a predetermined current while theinput voltage has the first polarity, (b) in the second direction at asecond rate determined by the value of the input voltage while the inputvoltage has the first polarity, (c) in the first direction at a thirdrate determined by the value of the input voltage, a predeterminedvoltage and the predetermined current while the input voltage has thesecond polarity and (d) in the second direction at a fourth ratedetermined by the value of the input voltage, the predetermined voltagewhile the input voltage has the second polarity, the predeterminedvoltage and predetermined current being of opposite polarity; and meansresponsive to the charging and discharging of the capacitor for derivingthe output frequency.
 8. The converter of claim 7 wherein the means forcyclically charging includes: means for supplying a predeterminedconstant current having the second polarity to the capacitor while thecapacitor is being charged in the second direction for a predeterminedinterval during each cycle and means for supplying a preset constantvoltage having the first polarity to the capacitor throughout each cyclewhile the input voltage has the second polarity, the magnitude of thepredetermined voltage being equal to the maximum current flowing fromthe source of the input voltage to the capacitor, the magnitude of thepredetermined current exceeding the magnitude of a maximum currentderived from the predetermined voltage.
 9. The converter of claim 7further including means for sensing the polarity of the input voltage,said voltage polarity sensing means being connected to said chargingmeans to control whether the capacitor is charged at the first andsecond rates or at the third and fourth rates and means for controllingthe digital signal deriving means so that the output frequency issampled at a predetermined rate.
 10. Apparatus for converting a bipolaranalog input voltage into a digital signal, comprising a polarity sensorresponsive to the analog input voltage, a voltage-to-frequency converterresponsive to the polarity sensed by the polarity sensor and the analoginput signal for deriving a variable frequency signal having doublevalues of frequency as a function of the magnitude of the analog inputsignal and its polarity and a discontinuity in the frequency derivedthereby as the analog input signal changes polarity, afrequency-to-digital converter responsive to the variable frequencyoutput of the voltage-to-frequency converter and of the polarity sensorfor deriving a digital signal having a polarity bit indicative of thesensed polarity and additional bits controlled by the frequency derivedby the voltage-to-frequency converter and the sensed polarity.
 11. Theapparatus of claim 10 wherein the voltage-to-frequency convertercomprises a charge balanced voltage-to-frequency converter, a referencevoltage source, and means responsive to the sensed polarity forselectively supplying current derived from the reference voltage sourceto the charge balanced voltage-to-frequency converter as a function ofthe sensed polarity.
 12. The apparatus of claim 11 wherein the chargebalanced voltage-to-frequency converter includes a constant currentsource selectively connected to an integrating capacitor of thevoltage-to-frequency converter during a portion of each cycle of theconverter, the voltage-to-frequency converter including impedance meansconnected to a source of the analog input voltage and the analog inputvoltage having a maximum magnitude such that a maximum predeterminedcurrent derived from the analog input voltage is supplied to thevoltage-to-frequency converter, the polarity of the current supplied tothe converter by the reference voltage source being different from thepolarity of the current supplied to the voltage-to-frequency converterderived from the constant current source, the magnitude of the currentsupplied to the voltage-to-frequency converter by the constant currentsource being greater than the sum of the current supplied to thevoltage-to-frequency converter by the reference voltage source plus thecurrent derived by the analog input voltage source.
 13. A circuit forconverting an input voltage having first and second opposite polaritiesto a digital representation of the input voltage, the input voltagehaving positive and negative values as well as zero, the digitalrepresentation being double valued such that (a) said representationincreases from a low value to a high value as the input voltage of thefirst polarity increases from zero to a full scale value, (b) thedigital representation decreases from said high value to said low valueas the input voltage of the second polarity increases from zero to saidfull scale value and (c) a polarity signal is generated indicating whichstate (a) or (b) applies, said circuit comprising:means for convertinginput voltages of the first polarity to corresponding digitalrepresentations; a plurality of sensing elements responsive to saidinput voltage for generating said polarity signal; a source of a biasvoltage equal to said full scale input voltage of the first polarity;and means for summing (1) a signal derived from the bias voltage with(2) a signal derived from the input voltage, when the input voltage isof the second polarity.